Certain types of fine-grained material, particularly certain types of lime and dolomite, have poor flow properties, especially in the ground state in which there is a high proportion of very fine material. The poor flow properties are especially prevalent in the case of soft types of limestone such as chalk or marly raw materials, and also in materials which decrepitate (i.e., disintegrate) in the firing process. The flow properties can be improved to some extent by coarser grinding. However, if the material has a tendency to decrepitation, the proportion of fine material builds up when the filter dust is delivered together with the fresh material to the hot gas pipe which leads to the last (uppermost) cyclone of a multi-stage cyclone preheater. Considerable operational disruptions then occur after a short time as a result of caking in the upper cyclones or because of the material flowing in batches through the apparatus. While in undisrupted normal operation the quantity of filter dust is approximately 5 to 10% of the quantity of fresh material, in materials which tend to break down into fine grains during calcination the filter dust can build up to over 40%. This gives rise to the poor flow properties of the material mentioned above, caking in the pipes, powder chutes and cyclones, and overloading of the upper cyclone stages. Under these conditions it is no longer possible for smooth operation to be maintained.
The object of the invention, therefore, is to provide a heat treatment method and apparatus in which satisfactory operation, without material caking and without material flowing in batches, is ensured even in the heat treatment of fine material which in the ground state has poor flow properties and/or has a tendency to decrepitation in the firing process.